1. Field of the Invention
The present invention relates to finely divided zirconia powders stabilized with yttrium values, and to an improved process for the production thereof.
2. Description of the Prior Art
Stabilized zirconia, and in particular zirconia stabilized with yttrium, are materials widely used in the preparation of ceramic compositions, notably to provide high mechanical, thermomechanical and electrical properties.
It is known to this art that the best mechanical and thermomechanical properties are provided by a monophasic zirconia consisting of metastable quadratic grains obtained by incorporating predetermined amounts of yttrium oxide into the zirconia lattice.
Dense shaped articles of stabilized zirconia are produced by sintering powders of an appropriate composition, but it is readily apparent that any improved properties thereof are directly proportional to the quality of the starting material powder itself. The production of a good ceramic shaped article from a poor powder is a problem that has remained elusive, even to this date.
More particularly, the initial powders must be of very high quality, i.e., highly reactive, of good purity and must be present in a quadratic form with a perfectly homogeneous distribution of the yttrium oxide within the zirconia.
These latter requirements are important such as not to produce, after sintering, a stabilized zirconia of the polyphasic type, i.e., having, in addition to the desired quadratic phase, more than an insignificant amount of parasite, in particular monoclinic phases.
On the other hand, from an industrial point of view, it is important that the synthesis of the powders be easy and readily carried out at temperatures as low as possible, and at an attractive cost.
Several processes are currently known to this art for the preparation of fine powders of stabilized zirconia.
One of the oldest processes consists of reacting, in the solid state, an intimate admixture of fine powders of zirconium oxide and an oxide of a stabilizing element, for example Y.sub.2 O.sub.3. However, this method has the disadvantage of being based on diffusion reactions between crystalline solids which are both slow and difficult, requiring very high temperatures and frequently involving the risks, on the one hand, of formation of heterogeneous solid solutions, and, on the other, of the introduction of undesirable impurities during the subsequent grinding stage, which is typically necessary after calcination.
More recently, it has been discovered that it is also possible to conduct a pyrolysis of precursors obtained by various different reactions which occur in the liquid phase.
Such processes generally described in the literature and based on this principle consist of mixing aqueous solutions of zirconium or a zirconyl salt with a salt of a stabilizing compound (sulfate, chloride, nitrate, etc.), then coprecipitating the two metals in the form of their hydroxides by addition of ammonia or an alkaline base, separating the coprecipitates from the resulting reaction medium, and pyrolyzing such coprecipitates.
The hydroxide precipitates are typically present in the form of amorphous gels, or aggregates of poorly crystallized fine crystals.
However, this type of method has several disadvantages. First, the hydroxide gels can absorb ions in more or less large amounts and these impurities, which are never eliminated completely in the washing of the precipitates, remain in the oxides after the dehydration heat treatment, with the exception of the sufficiently volatile species.
Secondly, the presence of water during the dehydration of the hydroxides favors the crystallization of the latter, such that their reactivity declines and the solid solution completely forms only at temperatures higher than 1,000.degree. C.
Finally, with this method it is not possible to control the size of the precipitates, such that the pyrolysis of the latter typically results in particles, the size distribution of which is relatively heterogeneous.
Another more favorable method entails pyrolyzing the solid precursors consisting of a homogeneous mixture or organic complexes of the carboxylate type (acetates, formates, citrates, and the like) of zirconium and the stabilizing compound, said precursors being produced by the evaporation of an aqueous mixture of the corresponding salts.
This method, described in published French Application No. 1,558,466, has the advantage of enabling the production of solid solutions at relatively low temperatures and directly in the form of fine powders.
Nonetheless, in following the teachings of FR No. 1,558,466 to produce a zirconia powder stabilized with yttrium, it has not been possible to prepare an essentially monophasic solid solution of the quadratic type.